The invention relates to a metal-ceramic substrate having at least one ceramic layer with first and second surface sides and a metallization provided on at least one of the surface sides, wherein the ceramic material forming the ceramic layer contains aluminium oxide, zirconium dioxide as well as yttrium oxide. The invention also relates to a method for producing a metal-ceramic substrate having at least one ceramic layer with first and second surface sides, wherein at least one of the surface sides is extensively bonded with at least one metallization wherein the ceramic layer is produced from a ceramic material including aluminium oxide, zirconium dioxide as well as yttrium oxide.
Metal-ceramic substrates in the form of printed circuit boards consisting of an insulator layer made from ceramic, and at least one metallization bonded to one of the surface sides of the insulator layer of ceramic and structured to form strip conductors, contacts, contact or attachment zones, are known in a wide variety of types.
Increasing the thermal conductivity of the ceramic layer is desirable in order to reduce the layer thickness of the metallization applied to a ceramic layer. In this regard, German Patent No. DE 10 2004 012 231 B4, for example, discloses a metallized ceramic substrate in which, in order to obtain a high mechanical strength and a high thermal conductivity, the ceramic layer is produced from a ceramic material containing a proportion of zirconium dioxide. In addition to aluminium oxide (Al2O3), the ceramic material contains zirconium dioxide (ZrO2) as well as yttrium oxide (Y2O3) and/or calcium oxide (CaO), wherein the Al2O3, ZrO2 and Y2O3 and/or CaO are contained in the ceramic layer in the following proportions with respect to the total weight of this layer:                Al2O3 between 91% and 97.96% by weight;        ZrO2 between 2% and 9% by weight; and        Y2O3 and/or CaO between 0.04% and 1% by weight.The thermal conductivity of a ceramic layer of that type is between 20 and 23 W/mK at room temperature. However, a further increase in the thermal conductivity would be desirable, while retaining or only slightly worsening the bending strength of the ceramic layer or the metallized ceramic substrate.        
The so-called “DCB process” (Direct Copper Bonding) is, for example, known for bonding metal layers or sheets, preferably copper sheets or films, to each other and/or to ceramic or ceramic layers, namely using metal or copper sheets or metal or copper films which have a layer or a coating (fusion layer) on their surface sides made from a chemical compound of the metal and a reactive gas, preferably oxygen. In that process described, for example, in U.S. Pat. No. 3,744,120 or German Patent No. DE 23 19 854, that layer or coating (fusion layer) forms a eutectic with a melting temperature below the melting temperature of the metal (for example copper) so that by depositing the metal or copper film on the ceramic and heating all of the layers, they can be bonded together by fusion of the metal or copper essentially only in the region of the fusion layer or oxide layer. A DCB process of this type thus comprises the following process steps, for example:                oxidizing a copper film in a manner so as to form a uniform layer of copper oxide;        depositing the copper film with the uniform copper oxide layer onto the ceramic layer;        heating the assembly to a process temperature between approximately 1025° C. and 1083° C., for example to about 1071° C.;        cooling to room temperature.        
Furthermore, documents German Patent No. DE 22 13 115 and European Patent No. EP-A-153 618 disclose the so-called active solder method for bonding metal layers or metal films which form metallizations, in particular copper layers or copper films, with a ceramic material or a ceramic layer. In that process, which in particular is also used for the production of metal-ceramic substrates, a compound between a metal film, for example copper film, and a ceramic substrate, for example an aluminium nitride ceramic, is formed at a temperature in the range about 800-1000° C. using a hard solder which, in addition to the major components such as copper, silver and/or gold, also contains an active metal. This active metal, which for example, is at least one element from the group Hf, Ti, Zr, Nb, Ce, by means of a chemical reaction, produces a bond between the hard solder and the ceramic, while the bond between the hard solder and the metal is a metallic hard solder bond.
From the prior art cited above, it is an object of the invention to provide a metal-ceramic substrate, and also an associated method for its production, which comprises a ceramic material containing a proportion of zirconium dioxide and with an improved thermal conductivity.